Various industrial production processes of acetic acid have been known. Among others, an industrially excellent process includes a process which comprises continuously allowing methanol to react with carbon monoxide with the use of a metal catalyst (such as a rhodium catalyst), an ionic iodide (e.g., lithium iodide), and methyl iodide in the presence of water to give acetic acid. Moreover, recently improvement in reaction conditions and catalysts was investigated, and an industrial process for producing acetic acid with a highly efficient production has been developed by addition of a catalyst stabilizer (such as an iodide salt) and the reaction under a low water content condition compared with the conventional condition.
Examples of the production process of acetic acid includes a process for producing purified acetic acid, which comprises allowing methanol to react with carbon monoxide, subjecting the resulting reaction mixture containing acetic acid to distillation (flash distillation) in a flash evaporator, subjecting a component vaporized by the distillation to a first distillation column to separate a liquid stream containing acetic acid as a main component and water, and others, subjecting the stream containing acetic acid to a second distillation column to remove water and others and separate an acetic acid stream as a liquid stream. In this process, condensation of hydrogen iodide in the first distillation column or the second distillation column may precipitate the corrosion of the distillation column. Since it is preferable the increase in concentration of hydrogen iodide in the distillation column be inhibited, the decrease in concentration of hydrogen iodide in the distillation column is being attempted.
For example, Japanese Patent Application Laid-Open No. 2006-160645 (JP-2006-160645A, Patent Document 1) discloses a process for distilling a mixture containing hydrogen iodide and water, which comprises distilling the mixture having a water content of not more than 5% by weight in a distillation system to prevent condensation of hydrogen iodide in the distillation system. With respect to a mixture applying the process, the document discloses that the process can be applied to alight component which is separated from the reaction mixture by a first distillation (distillation by a flash evaporator or the like) and is rich in a low boiling point component (e.g., water, an alcohol, an alkyl iodide, a carboxylic acid or an acid anhydride thereof, a carboxylate ester, and hydrogen iodide). In the process described in the document, however, the concentration of hydrogen iodide is reduced by adjusting the concentration of water based on the equilibrium theory, and there are limitations to the decrease in the concentration of hydrogen iodide. Thus it is difficult to decrease the concentration of hydrogen iodide at a high level. Moreover, since the process described in the document is applied to a light component obtained through a flash distillation, the condensation of hydrogen iodide in a distillation column for further purifying acetic acid separated from the light component cannot be inhibited.
Moreover, Japanese Patent Application Laid-Open No. 2009-501129 (JP-2009-501129A, Patent Document 2) discloses a process for producing purified acetic acid, which comprises feeding an acetic acid stream containing acetic acid, a hydrogen halide, a lower boiling point component and a higher boiling point component to a first distillation column, separating a lower boiling point stream containing part of the lower boiling point component and a higher boiling point stream containing part of the higher boiling point component in the first distillation column, withdrawing a side stream containing at least acetic acid by side cut, feeding the side stream to a second distillation column, separating a lower boiling point stream containing part of the lower boiling point component and a higher boiling point stream containing part of the higher boiling point component in the second distillation column, and withdrawing a side stream containing acetic acid by side cut to collect (or recover) acetic acid; which further comprises feeding (i) the first distillation column with water, or water and at least one first component (A) selected from the group consisting of an alcohol corresponding to the carboxylic acid and having “n” carbon atom(s), and an ester of the alcohol with the carboxylic acid or (ii) the first distillation column with the first component (A) with from a height level (position) lower than a side stream port for conducting side cut of the side stream containing the carboxylic acid having “n+1” carbon atoms.
This document discloses that at least one second component (B) selected from the group consisting of (b-1) methanol, (b-2) methyl acetate, (b-3) an alkali metal hydroxide (e.g., potassium hydroxide), (b-4) an alkali metal acetate (e.g., potassium acetate), and (b-5) a hypophosphorous acid can usually be fed to the second distillation column from at least one position upper or lower than a side cut port (side stream port) for conducting side cut of the acetic acid stream in order to reduce the concentration of hydrogen iodide contained in the acetic acid stream withdrawn by side cut and prevent the condensation of hydrogen iodide in the distillation column. In Examples of the document, potassium hydroxide is fed to the second distillation column at a height level (or plate) lower than the side stream port or at a height level (or plate) which is upper than the side stream port and lower than a feed port for feeding the acetic acid stream to the second distillation column.
The process described in the document achieves the condensation of hydrogen iodide in the second distillation column to some extent. However, hydrogen iodide is contained in the acetic acid stream to be fed to the second distillation column and is moved together with water to an upper part (or top) of the second distillation column by distillation, while potassium hydroxide or the like is moved downward. Therefore, when potassium hydroxide is fed to the second distillation column at a height level lower than a port for feeding the acetic acid stream to the second distillation column as described in the document, it is difficult to efficiently inhibit the condensation of hydrogen iodide in the upper part (or top) of the second distillation column. Moreover, in the process described in the document, although the second component (B) is fed in order to decrease hydrogen iodide contained in the side cut stream fed from the first distillation column, in an actual system not only hydrogen iodide contained in the side cut stream but also hydrogen iodide newly produced by a reaction of methyl iodide with water, and other reactions in the upper part of the second distillation column exists in the second distillation column. The condensation of hydrogen iodide existing in the upper part of the distillation column cannot be inhibited using potassium hydroxide or the like efficiently. According to the process described in the document, even if the quality of acetic acid can be improved by reducing the concentration of hydrogen iodide (HI) contained in the side cut stream, it is difficult to reduce the concentration of hydrogen iodide at a high level in the whole second distillation column. Further, the concentration of hydrogen iodide in the whole second distillation column may be reduced by feeding an alkali metal hydroxide (e.g., potassium hydroxide) at a height level lower than the feed port of the second distillation column and feeding an alcohol (e.g., methanol) at the same time (or the same position) the alkali metal hydroxide or at a position lower than the feeding position of the alkali metal hydroxide. In this case, however, a distillation column having a large column diameter is necessary, and the process is inefficient.
Incidentally, Japanese Patent Application Laid-Open No. 48-61414 (JP-48-61414A, Patent Document 3) discloses a method for removing (or separating) iodine from acetic acid, which comprises introducing an acetic acid stream containing iodine as an impurity into a middle part of both ends of a first distillation column, introducing an alkali metal or alkaline earth metal compound (an oxide, a hydroxide, a carbonate, a bicarbonate, or a weak organic acid salt of an alkali metal or alkaline earth metal) into the middle part of both ends of the first distillation column, withdrawing an overhead product stream from the first distillation column, introducing the product stream into a middle part of both ends of a second distillation column, and withdrawing an acetic acid stream substantially free from iodine from a lower part of the second distillation column and withdrawing an overhead fraction containing iodine from the second distillation column.
According to the method described in the document, the alkali metal or alkaline earth metal compound is fed to the product stream from the first distillation column or the second distillation column. However, the product streams are overheads in the first and second distillation columns, and this method is quite different in production process of acetic acid from the above-mentioned process for separating the acetic acid stream as a liquid component. For example, the method described in the document intends to decrease hydrogen iodide contained in a purified product stream, and the method is quite different in liquid to be treated from the above-mentioned process.